tailieunhanh - Kỹ thuật truyền số liệu 9

Tham khảo tài liệu 'kỹ thuật truyền số liệu 9', giáo dục - đào tạo, cao đẳng - đại học phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | P I ER 9 Error Detection and Correction Networks must be able to transfer data from one device to another with complete accuracy. A system that cannot guarantee that the data received by one device are identical to the data transmitted by another device is essentially useless. Yet anytime data are transmitted from source to destination they can become corrupted in passage. In fact it is more likely that some part of a message will be altered in transit than that the entire contents will arrive intact. Many factors including line noise can alter or wipe out one k or more bits of a given data unit. Reliable systems must have a mechanism for detecting and correcting such errors. Data can he corrupted dtirine transmission. For reliable communication. errors must be detected and corrected. Error detection and correction are implemented either at the data link layer or the transport layer of the OSI model. TYPES OF ERRORS Whenever an electromagnetic signa flows from one point to another it is subject to unpredictable interference from heat magnetism and other forms of electricity. This interference can change the shape or timing of the signal. If the signal is carrying encoded binary data such changes can alter the meaning of the data. In a single-bit error a 0 is changed to a 1 or a 1 to a 0. In a burst error multiple bits are changed. For example a burst of impulse noise on a transmission with a data rate of 1200 bps might change all or some of 12 bits of information see Figure . Single-Bit Error The term single-bit error means that only one bit of it given data unit such as a byte character data unit or packet is changed from 1 to 0 or from 0 to 1. 273 274 CHAPTER 9 ERROR DETECTION AND CORRECTION Figure Types of errors In a single-bit error only one bit in the datti unit has changed. Figure shows the effect of a single-bit error on a data unit. To understand the impact of the change imagine that each group of eight bits is an ASCII .